Fastmatte is lean, and 50:50 Galkyd/Gamsol is lean. The idea of fat over lean with alkyds – you don't need to worry too much – just add less and less 50:50 mixture of Galkyd/Gamsol as you add fastmatte layers.
Then, when switching to oils over the Fastmatte layers, use a 50:50 mixture of Cold wax medium to Galkyd as your THINNING medium.
Galkyd – is fat
Gamsol – is lean
When you make a 50:50 mixture, it cuts down on the yellowing effect of the galkyd.